1. Field of the Invention
The present invention relates to a production method for a flexible printed board.
2. Description of the Art
Conventional flexible printed boards generally have a three-ply laminate structure such that a copper foil is stacked on a polyimide resin film with the intervention of an adhesive layer. With recent advancement of the electronic industry, the flexible printed boards are increasingly demanded to have a high temperature resistance. However, the adhesive layer of such a flexible printed board of three-ply laminate structure is poor in heat resistance. To cope with this, there is proposed a flexible printed board having a two-ply laminate structure such that a copper foil is directly stacked on a polyimide resin film.    Exemplary production methods (1) to (3) for the flexible printed board of two-ply laminate structure will be described below.    (1) A polyimide varnish is applied onto a surface of a copper foil, and then heated.    (2) After a surface of a polyimide resin film is plasma-treated in a vacuum vessel, nickel or chromium is sputtered on the surface of the polyimide resin film for formation of a foundation, and copper is sputtered on the foundation. After the resulting polyimide resin film is taken out of the vacuum vessel, the polyimide resin film is electroplated with copper.    (3) A surface of a polyimide resin film is etched with an aqueous solution of a mixture of hydrazine and an alkali metal hydroxide, and then electrolessly plated with nickel or cobalt. In turn, the resulting polyimide resin film is heat-treated at a high temperature on the order of 400° C., and then electro-plated with copper (Japanese Unexamined Patent Publication No. 5-114779 (1993)).
In the case of the method (1), however, it is impossible, from the viewpoint of the workability, to employ a copper foil having a thickness of not greater than 12 μm. In the case of the method (2), the surface modification of the polyimide resin film by the plasma treatment alone is not sufficient to change the surface configuration of the polyimide resin film. Therefore, a physical anchoring effect cannot be expected, so that the adhesion of the copper film is weak. In addition, the use of the vacuum vessel leads to higher equipment costs and difficulty in continuous production, resulting in higher costs. In the case of the method (3), the use of highly deleterious hydrazine deteriorates the workability. In addition, a high temperature heat treatment should be performed between the electroless plating step and the electroplating step for imidizing a part of polyimide affected by intensive etching. This makes the continuous production difficult.
In view of the foregoing, it is an object of the present invention to provide a production method for a flexible printed board, which ensures higher adhesion of copper, better workability, easier continuous production and lower costs.